1. Field of the Invention
The present invention relates generally to an antenna corresponding to a plurality of radio communication frequencies, and more particularly, to an antenna apparatus and a radio communication apparatus including a passive element preferred for various radio communications such as a folding cellular phone.
2. Description of the Related Art
In a mobile telephone service such as a cellular phone, radio communication frequencies are increased and made multiband such as dual-band or triple-band as a coverage area is expanded, and an antenna corresponding to such frequencies is needed.
A radiant efficiency is an evaluation item of antenna characteristic and is improved relatively easily by a distance between an antenna and ground (GND) or by a matching circuit. Since a cellular phone is generally used by bring a receiver into contact with ear and the antenna comes closer to a human head, radiated electric waves are attenuated by the effect of a human body in no small way. As the extent of the attenuation of the radiated electric waves is increased, the radio communication may be disturbed. Since a plurality of resonance points exists in one antenna in the case of a multiband antenna, the radiant efficiency tends to be inferior to a single-band antenna.
Such an antenna apparatus with a plurality of resonance points includes Japanese Patent Application Laid-Open Publication Nos. 1998-190347, 1998-190344, 2004-304705, 2004-128557, etc.; Japanese Patent Application Laid-Open Publication No. 1998-190347 discloses a patch antenna apparatus corresponding to a plurality of frequencies, which is provided with a plurality of additional patch units connected by a PIN diode on a patch unit and which makes the PIN diode conductive and nonconductive to switch connection of the patch unit and the additional patch units; Japanese Patent Application Laid-Open Publication No. 1998-190344 discloses an antenna with a feed element and a passive element connected by a switching element, which changes a resonance frequency by opening and closing the switching element; Japanese Patent Application Laid-Open Publication No. 2004-304705 discloses a radio apparatus including a slot electromagnetically coupled to an antenna as well as a switch for shorting or opening a bottom board region of the slot, which switches radiation patterns of the antenna by opening and closing the switch; and Japanese Patent Application Laid-Open Publication No. 2004-128557 discloses an antenna including first and second antennas on a printed board, which changes electric length of the antennas to switch directional characteristics.
By the way, in a radio communication apparatus, such as a cellular phone, coming closer to a human body at the time of communication, since the radiated electric waves of the antenna are affected by a human body in no small way and the effect thereof cannot be eliminated completely, an antenna apparatus corresponding to the multiband is needed. That is, an antenna apparatus is requested which enhances the radiant efficiency, which reduces the effect of a human body, and which is suitable for the multiband.
Japanese Patent Application Laid-Open Publication Nos. 1998-190347, 1998-190344, 2004-304705, and 2004-128557 do not disclose or indicate such problems and do not disclose or include a concept of an antenna apparatus that solves the problems.